The present invention relates to a method and apparatus for automatic range finding which is used for an automatic focussing camera.
Almost all recent compact cameras are provided with means for determining automatically the distance from the subject to be photographed to the camera and for adjusting the focus of the camera lens according to that determination. The means for determining the subject distance sights as a target a small area at the center of the field of view of the camera lens. A target mark indicates that small central area in the viewfinder. The target mark must be aligned with the subject which is intended to be photographed in order to determine automatically accurately the subject distance of the intended subject. For an actual subject distance determination, after aiming the target mark at the intended subject to be photographed, an operating member, which is usually a shutter release member, is depressed halfway so as to actuate the subject distance determining means. If the shutter release member is further depresed, the adjustment of the focus of the camera lens is effected in accordance with that determined subject distance and then the shutter is actuated.
Conventional compact cameras provided with the automatic subject distance determining means described above require aiming the target at the subject intended to be photographed. For this reason, if the subject on which the camera lens is to be focussed is eccentric of the view field of the camera finder, it is necessary to orient the camera so as to direct the target mark to the intended subject in order to effect the desired subject distance determination prior to framing. Then, to effect framing after the subject distance determination, it is necessary to reorient the camera while keeping the shutter release member depressed halfway.
Because of this difficult subject distance determination, the conventional compact cameras are neither quick nor easy to use. In order to remedy this, there has been proposed a compact camera which has means for automatically determining the subject distance by sighting as a target a wide area of the field of view of the camera lens, that is, without the provision of a target mark in the camera finder. This subject distance determining means with no target mark (which is hereinafter referred to as a target-free subject distance determining device) is disclosed in Japanese Patent Appln. No. 60-87,894 having the same assignee as this application. The target-free subject distance determining device comprises light projecting means for emitting a transverse line of illumination light toward a subject and light detecting means comprising a large number of small photoelectric conversion elements (which are hereinafter referred to as pixels) arranged in m lines.times.n columns for detecting the light reflected from the subject. In the target-free subject distance determination device, the determination of subject distance is made according to a pixel line comprised of pixels receiving the reflected light.
Although the above-described target-free subject distance determining device obviates the necessity of reorienting the camera after a subject distance determination, there remains the problem of double-subject detection due to the use of a line of illumination light. Specifically, if there are two subjects at different distances in the field of view of the camera lens, the reflected light from those subjects is detected by at least two pixels in different pixel lines. The target-free subject distance determining device cannot decide which one is detecting the subject actually intended to be photographed. For avoiding such confusion from double-subject detection, the target-free subject distance determining device is adapted to adjust the focus of the camera lens to the nearer subject. Therefore, if a subject which is intended to be photographed is in fact farther from the camera than the other subject in the field of view of the camera lens, the camera lens will not focus on the intended subject.
For example, when the intended subject is at a distance less than, say, four meters, which is the limit of a predetermined short photographic distance range, and the subject is approximately at the center of image field, but farther from the camera than another subject off to the side of the intended subject, the camera lens is adjusted to that other subject, with the result that the intended subject image is out of focus and loses sharpness. This loss of sharpness of the intended subject can be ignored when both subjects are beyond the short photographic distance range, namely, in a long photographic distance range wherein ordinary camera lenses are sufficiently focussed on subjects at any in-range distances. But the loss of sharpness becomes too great to ignore within the short distance photographic range.